There is a family of high activity vanadium catalysts that have been described for the polymerization of olefins such as ethylene and .alpha.-olefins, that are based on the use of a supported reduced and complexed vanadium halide catalyst precursor. Illustrations of these catalysts can be found in Beran et al., U.S. Pat. No. 4,508,842, patented Apr. 2, 1985. Beran et al. describe an ethylene polymerization catalyst comprising a supported precursor of a vanadium halide/electron donor complex and alkylaluminum or boron halides to form a reduced vanadium catalyst precursor, which when combined with alkylaluminum cocatalyst and alkyl halide promoter, provides enhanced polymerization and productivity plus a superior polyethylene product. According to Beran et al., the process involves polymerizing ethylene with or without at least one C.sub.3 to C.sub.10 alpha-olefin monomer in the gas phase at a temperature between about 30.degree. C. to about 115.degree. C. wherein the monomers are contacted with a catalyst composition comprising a vanadium complex and a modifier which are impregnated on a solid, inert carrier. Beran et al. differentiate by the use of a supported precursor, a cocatalyst and a promoter in which the supported precursor comprises a vanadium halide-electron donor reaction product and modifier impregnated on a solid, inert carrier. The reaction between the vanadium halide-electron donor complex and the modifier is described as a single stage reaction and, thus, such a vanadium catalyst precursor is herein termed a "non-stage-modified" vanadium catalyst precursor. The halogen in the vanadium halide is chlorine, bromine or iodine, or mixtures thereof. A particularly preferred vanadium halide is a vanadium trihalide, such as vanadium trichloride, VCl.sub.3. The electron donor is a liquid, organic Lewis base in which the vanadium halide is soluble. The electron donor is selected from the group consisting of alkyl esters of aliphatic and aromatic carboxylic acids, aliphatic ketones, aliphatic amines, aliphatic alcohols, alkyl and cycloalkyl ethers, and mixtures thereof. Preferred electron donors are alkyl and cycloalkyl ethers, such as tetrahydrofuran ("THF"). Between about 1 to about 20, preferably between about 1 to about 10, and most preferably about 3 moles of the electron donor are complexed with each mole of vanadium used.
The disclosure of Beran et al. is incorporated herein by reference. The vanadium catalysts of Beran et al. are hereinafter characterized as the "Beran et al. Catalyst System."
There is substantial literature indicating the creation of a catalytically active vanadium by the reduction of vanadium halides to a reduced, viz. divalent, state. Carrick et al., JACS, vol. 82, p. 1502 (1960) describe the reduction of VCl.sub.4 to the divalent state form of a vanadium ethylene catalyst utilizing conventional reducing agents, such as triisobutylaluminum and zinc alkyls. Karol et al., JACS, vol 83, pp. 2654-2658 (1961) discuss the partial and total reduction of vanadium halides such as VCl.sub.4 to divalent structures and the catalystic activity resulting with respect to the polymerization of ethylene to polyethylene.
Jacob et al., Z. anorg. allg. Chem., 427, pp. 75-84 (1976) illustrate the complexity of such reduction reactions in the presence of THF. From the teachings of Beran et al., the resulting divalent vanadium compounds are complexes which include THF in the structure.
Cumulative to the above, Smith et al., U.S. Pat. No. 4,559,318, patented Dec. 17, 1985, describe a number of procedures for making VX.sub.2, where X is halogen, which involves the reduction of VX.sub.4 or VX.sub.3 by reaction with reducing agents followed by the complexation of the VX.sub.2 with an ether such as THF. Such is effected on a support surface.
A number of significant problems have been noted with the fluid bed operability of high activity vanadium catalysts, including the vanadium catalyst encompassed by the Beran et al. Catalyst System, in the polymerization of ethylene and .alpha.-olefins to produce elastomeric polymers, such as ethylene-propylene copolymers (EPR). These problems are oftentimes characterized by the formation of polymer chips, chunks, sheets and lumps in the fluid bed, which in some cases can lead to sudden defluidization. Another set of problems can occur with high activity vanadium catalysts on starting up of the fluid bed and/or during transitioning of the reactor. For example, start-up of a gas-fluidized polymerization reactor goes through a sensitive stabilization period due to impurities trapped in the reaction system. Low (ppm) level impurities have a deactivating influence on the catalyst and contribute to polymer particle adhesion. The net effect is that layers of polymer fines containing high concentrations of catalyst are formed on reactor surfaces and in places where mixing forces are reduced. When polymerization is then initiated, localized hot spots can result, with consequent chunking and eventual reactor shutdown. Therefore, it is desirable to have a catalyst that has a gradual kinetic profile which minimizes sudden acceleration of polymerization as reaction is initiated.
Although such difficulties have occurred in a variety of ethylene polymerization operations with such catalysts, they have been most pronounced under EPR operating conditions, especially under EPM operating conditions, where there are relatively high concentrations of propylene in the reactor and a low-crystallinity resin is being produced. In this case, these difficulties are believed to stem from a number of contributing factors, such as (i) the magnitude of the initial kinetic spike in the standard catalyst reaction profile, which is much greater with propylene as a comonomer than with .alpha.-olefins higher than propylene and (ii) the elastomeric nature of the EPM resin being produced, which can soften, become sticky and agglomerate due to the increase in temperature associated with hot spots and reaction surges.
Important variables in influencing the degree of stickiness leading to more or less agglomeration are the polymerization reaction temperature and crystallinity of the polymer being produced. Higher temperatures increase the propensity to form agglomerates, and less crystalline polymers, such as ultra low density polyethylene, ethylene/propylene copolymers (EPM), and ethylene/propylene/diene monomer (EPDM), usually display a greater tendency to agglomerate. EPM and EPDM polymers having a density less than 0.88 g/cc are noted for their capacity to soften and agglomerate.
Those polymerization conditions which result in stickiness and agglomeration of the polymer are termed "polymerization conditions that normally would yield an undesirable amount of agglomerated polymer with non-stage-modified high activity vanadium catalysts," in order to characterize this invention so as to compensate for the variety of reactants, polymerization conditions and catalyst compositions encompassed herein.
Elastomeric ethylene-alpha-C.sub.3 -C.sub.18 olefin copolymers encompass ethylene-propylene copolymers (EPR) (inclusive of EPM or EPDM copolymers), ethylene-butene copolymers, and the like. Illustrative of such polymers are those comprised of ethylene and propylene or ethylene, propylene and one or more dienes. Copolymers of ethylene and higher alpha-olefins such as propylene often include other polymerizable monomers, such as non-conjugated dienes, illustrated by the following:
straight chain acylic dienes such as: 1,4-hexadiene, 1,6-octadiene, and the like; PA1 branched chain acyclic dienes such as: 5-methyl-1,4-hexadiene, 3,7-dimethyl-1,6-octadiene, 3,7-dimethyl-1,7-octadiene and the mixed isomers of dihydro-myrcene, dihydroocinene, and the like; PA1 single ring alicyclic dienes such as: 1,4-cyclohexadiene, 1,5-cyclooctadiene, 1,5-cyclododecadiene, and the like; PA1 multi-ring alicyclic fused and bridged ring dienes such as: tetrahydroindene, methyltetrahydroindene, dicyclopentadiene, bicyclo(2,2,1)-hepta-2,5-diene, alkenyl, alkylidene, cycloalkenyl and cycloalkylidene norbornenes such as 5-methylene-2-norbornene (MNB), 5-ethylidene-2-norbornene (ENB), 5-propyl-2-norbornene, 5-isopropylidene-2-norbornene, 5-(4-cyclopentenyl)-2-norbornene, 5-cyclohexylidene-2-norbornene, and the like.
Of the non-conjugated dienes typically used to prepare these copolymers, dienes containing a double bond in a strained ring or .alpha. position, are preferred. The most preferred dienes are 5-ethylidene-2-norbornene (ENB) and 1,4-hexadiene. The amount of diene, on a weight basis, in the copolymer can range from about 0% to about 20% with about 0% to about 15% being preferred. The most preferred range is 0% to 10%.
The preferred EPR copolymers are copolymers of ethylenepropylene (EPM) or ethylene-propylene-diene (EPDM). The average ethylene content of the copolymer could be as low as about 10% on a weight basis. The preferred minimum is about 25%. A more preferred minimum is about 30%. The maximum ethylene content may be about 85% on a weight basis. The preferred maximum is about 80%, with the most preferred maximum being about 75 weight % ethylene.
High activity vanadium catalysts would be desirable for EPR products because they achieve efficient comonomer incorporation in the polymers with relatively random distribution of the comonomers in the polymer structure. However, their above-noted deficiencies for producing EPR polymers in a fluid bed, such as producing a high initial surge in the polymerization rate which causes exothermic temperature excursions that can soften the polymer in the course of polymerization and foul the bed by virtue of resin agglomeration, thereby degrading fluid bed operability, has impaired their use for such applications. The initial surge is strongest when propylene is one of the comonomers polymerized with ethylene and is present in relatively high concentrations. The problem is magnified in EPR copolymers because of their low softening temperature. Thus, there is a need for a high activity vanadium-based catalyst system that can produce EPR-type polymers at acceptable productivity levels without inducing agglomeration. There is also a need for a high activity vanadium catalyst system that is effective in producing a variety of ethylene-containing polymers with low softening temperatures.
As part of the prior art to the invention, commercial production of a copolymer of ethylene and 1-butene was practiced using a stage-modified Beran et al. Catalyst System, as described hereinafter, under conditions where the properties of the copolymer were a density of 0.898 g/cc. and a melt index of 0.47 dg/min. It was made concurrently with the same copolymer of ethylene and 1-butene employing a straight (non-stage-modified) Beran et al. Catalyst System under the same operating conditions. No difference in the operation of polymerization reactions with the two catalysts were anticipated and none were found. The lack of difference shows that the conditions were not such as to cause reaction surges leading to agglomeration of the polymer being produced.
It would be extremely beneficial to conduct polymerizations of elastomeric resins at temperatures close to or even above the softening point of the sticky polymer, since it is well known that increases in polymerization temperature generally enhance the yield of product in relation to the catalyst, and, in addition, more economical removal of the heat of the reaction is achievable and purging of residual unreacted monomers from the product becomes more efficient. Therefore it is desirable to have a catalyst that has a gradual kinetic profile which minimizes any sudden acceleration of the reaction as polymerization is initiated with that catalyst.